1. Field of the Invention
This invention relates generally to Java programming, and more particularly to methods for managing replicated and migration state in a Java environment.
2. Description of the Related Art
Today's world of computer programming offers many high-level programming languages. Java, for example, has achieved widespread use in a relatively short period of time and is largely attributed with the ubiquitous success of the Internet. The popularity of Java is due, at least in part, to its platform independence, object orientation and dynamic nature. In addition, Java removes many of the tedious and error-prone tasks that must be performed by an application programmer, including memory management and cross-platform porting. In this manner, the Java programmer can better focus on design and functionality issues.
One particular Java environment is the Java 2 platform, Enterprise Edition (J2EE), which facilitates building Web-based and enterprise applications. Broadly speaking, J2EE services are performed in the middle tier between the user's browser and the databases and legacy information systems. J2EE comprises a specification, reference implementation and a set of testing suites. J2EE further comprises Enterprise JavaBeans (EJB), JavaServer Pages (JSP), Java servlets, and a plurality of interfaces for linking to information resources in the platform.
The J2EE specifications define how applications should be written for the J2EE environment. Thus the specifications provide the contract between the applications and the J2EE platform. One aspect of the J2EE specification is the EJB 2.0 Container Managed Persistence (CMP). The EJB 2.0 specification defines a contract between an entity bean, its container and the persistence manager for the management of persistent state and relationships for the entity beans. For a complete specification of CMP, refer to the EJB 2.0 specification published by Sun Microsystems, Inc., which is incorporated by reference herein in its entirety.
According to the EJB programming model, a bean provider develops a set of entity beans for an application and specifies the relationships between these objects. For each entity bean, the bean provider specifies an abstract persistence schema, which defines a set of methods for accessing the container-managed fields and relationships for the entity bean. The container-managed fields and relationships of the abstract persistence schema are specified in the deployment descriptor defined by the bean provider.
The deployer uses the persistence manager provider tools to determine how persistent fields and relationships are mapped to the underlying persistence mechanism, such as, a database. The persistence manager tools also generate the additional classes and interfaces that enable the persistence manager to manage the persistent fields and relationships of the entity beans at the runtime. An advantage of container managed persistence is that the entity beans become logically independent of the underlying persistence mechanism. The CMP also leads to a simple programming mode for managing persistence.
An entity bean with container manager persistence includes its class, a remote or local interface that defines its client-view business methods, a home interface that defines create, remove, home and finder methods. The abstract persistence schema comprises a set of properties, each representing a field or relationship in the persistent state of entity bean. The entity bean defines a set of accessor (setter and getters) methods for the persistent fields and relationships.
The bean provider generally does not write any database access calls in the entity bean class. Instead, a persistence manager that is available to the container at runtime handles the persistence. The bean provider codes all persistent data access using the setter and getter methods defined for the container-managed persistent and relationship fields.
FIG. 1 is a diagram showing the tradeoffs that business and carrier grade applications have to make. The carrier grade applications, which are applications used in high-performance, high-traffic networks such as used by telecoms, service providers and ISPs, require higher performance and availability than business applications.
Carrier grade (CG) applications require high availability of the order of five 9's and better. A high availability environment is one in which a service or component has greater availability, usually due to component redundancy, than in some base environment. Typically the term is used to describe failover cluster environment in which a service is provided by a primary component, until after some failure, after which the secondary component takes over the provision of the service. The high availability requirement for carrier grade applications leads to a requirement for a carrier grade application to achieve prompt (two seconds or less) restart or failover to a secondary component with minimal disruption of service. Thus, the application has to become operational within minimal time after a failure.
A carrier grade application requires a shorter failover time as compared to the business applications, which typically store persistent state in a database to achieve ACID (Atomicity, Consistency, Isolation, Durability) properties for the managed state. Business applications rely on database-specific mechanisms to achieve state replication, thereby protecting persistent data from the failure of the primary database.
A typical database replication mechanism is database log replay. The database logs changes in a transaction log that is used for transaction replay in case of a failure. The log replay involves applying the transaction log to a replica database so that a near-mirror copy of primary database is created. Unfortunately, the time-delay in replaying the transaction log on the replica database slows down the failover. Moreover, a huge rate of inserts/updates may create huge transaction replay log, which further slows the transaction replay at the failure time.
Business applications also rely on parallel database servers to achieve state replication and failover. In case of parallel database servers, multiple active database engines, coordinated by a distributed lock manager, manage active replication. However, use of the distributed lock manager for coordinating database operations slows down the application performance. Hence, the use of a parallel database server is considered more suitable for read-only and read-mostly applications. With the short failover time requirement, a typical carrier grade application cannot rely on a database-based replication mechanism or a parallel database server to achieve state replication and failover.
Business applications typically need to maintain ACID properties for the data being used by the application. Such applications cannot afford any data inconsistency and thereby store persistent data in a database and use transactions. For business applications, the tradeoff between consistency and concurrency gets reflected in the choice of the database isolation level. The use of optimistic concurrency model as against pessimistic concurrency model is another design decision involved in the business applications as part of the consistency and concurrency tradeoff. Business applications also need a reliable and consistent database failover and recovery—these applications cannot afford any inconsistent data.
The carrier grade applications may have consistency and concurrency requirements that differ from the business applications. A carrier grade application may process multiple concurrent state transitions and may not require full ACID properties to be maintained for its managed state. For example, a CG J2EE application may need a fast failover and may afford to have the replica (now the new primary) take over in a state that is temporally inconsistent with the initial primary. The client using the carrier grade application should be able to bring the new primary into a consistent state by retrying transactions for state transitions.
In view of the forgoing, there is a need for systems and methods that manage state for applications that require high availability. The systems and methods should allow for state replication and provide high availability of the order of five 9's and better to carrier grade applications.